At times it is necessary to operate hydraulic turbines outside the range of conditions for which the turbine was designed. This is commonly referred to as "off peak operation." Off peak operation causes power swings in the system and can result in damaging vibration to the machine.
These power swings are attributable to or at least related to pressure surges in the draft tube. It is generally felt that the pressure surges are due to a rotating vortex at the runner exit into the draft tube which is mainly a function of the angular momentum of the water leaving the runner. The vortex is in effect a low pressure area in the draft tube.
It is known that the spiral-shaped vortex rotates at approximately one-third of the runner speed. When this takes place the pressure distribution at most horizontal cross sections of the draft tube is not uniform. At the point of cross sectional circumference of the draft tube to which the core of the vortex is closest the pressure is the lowest. Conversely, the point of cross sectional circumference of the draft tube farthest from the core of the vortex experiences the highest pressure. These uneven pressures at the draft tube wall supply resultant forces perpendicular to the vortex which causes the vortex to rotate.
In the past it has been found beneficial to inject air at various locations in the turbine including the draft tube. It was hoped that the air would mix sufficiently with the water to create a constant pressure in the draft tube thereby dissipating the pressure surges. Examples of such prior art structure are disclosed in U.S. Pat. No. 1,848,738 issued to L. F. Moody on Mar. 8, 1932 and German Pat. No. 279,351 having a data of Feb. 7, 1914.
In some instances the prior art method of uniformly injecting air into the draft tube did produce improved results. However, in many other cases it did not. Furthermore, in many cases the cost of the equipment to supply the necessary amount of air was greater than the savings resulting from dissipating the pressure surges.